Add util function to locate a column in a string

This will help to parse the result of "adb shell ip route" to find the
device IP address.

PR #2827 <https://github.com/Genymobile/scrcpy/pull/2827>

app/src/util/str.c

@@ -304,3 +304,29 @@ sc_str_truncate(char *data, size_t len, const char *endchars) {
     data[idx] = '\0';
     return idx;
 }
+
+ssize_t
+sc_str_index_of_column(const char *s, unsigned col, const char *seps) {
+    size_t colidx = 0;
+
+    size_t idx = 0;
+    while (s[idx] != '\0' && colidx != col) {
+        size_t r = strcspn(&s[idx], seps);
+        idx += r;
+
+        if (s[idx] == '\0') {
+            // Not found
+            return -1;
+        }
+
+        size_t consecutive_seps = strspn(&s[idx], seps);
+        assert(consecutive_seps); // At least one
+        idx += consecutive_seps;
+
+        if (s[idx] != '\0') {
+            ++colidx;
+        }
+    }
+
+    return col == colidx ? (ssize_t) idx : -1;
+}

app/src/util/str.h

@@ -114,4 +114,24 @@ sc_str_wrap_lines(const char *input, unsigned columns, unsigned indent);
 size_t
 sc_str_truncate(char *data, size_t len, const char *endchars);
 
+/**
+ * Find the start of a column in a string
+ *
+ * A string may represent several columns, separated by some "spaces"
+ * (separators). This function aims to find the start of the column number
+ * `col`.
+ *
+ * For example, to find the 4th column (column number 3):
+ *
+ *     //                               here
+ *     //                               v
+ *     const char *s = "abc def    ghi  jk";
+ *     ssize_t index = sc_str_index_of_column(s, 3, " ");
+ *     assert(index == 16); // points to "jk"
+ *
+ * Return -1 if no such column exists.
+ */
+ssize_t
+sc_str_index_of_column(const char *s, unsigned col, const char *seps);
+
 #endif